Back from the dead It may be the ultimate paternity test for a reptile believed lost to history.

US scientists say an iconic tortoise that has been presumed extinct in the Galapagos Islands for 150 years may still exist, based on DNA blood samples from the giant creatures' living children.

The reptile in question is a majestic tortoise known as Chelonoidis elephantopus, which can weigh up to 400 kilograms and live for a century in the wild.

However, they were only known to exist on Floreana Island in the Galapagos and were presumed extinct shortly after Charles Darwin's historic voyage there in 1835.

But researchers at Yale University have sampled DNA from 2000 tortoises of a related species, C. becki, on nearby Isabella Island, and found what they say are unmistakable traces of C. elephantopus in their parentage.

Eighty-four of the tortoises have DNA that indicates their ancestry is a mix between C. becki and C. elephantopus, according to research published in the latest edition of Current Biology.

By comparing the living hybrids' DNA to that in museums, "the newly sampled individuals can only be explained if one of their two parents were C. elephantopus," the researchers write.

Since the lumbering tortoises are land-bound reptiles, humans may have transferred them from island to island via ship, they add.

However, lead author Ryan Garrick says it would take quite a stroke of luck to come across an actual C. elephantopus.

"To our knowledge, this is the first report of the rediscovery of a species by way of tracking the genetic footprints left in the genomes of its hybrid offspring," says Garrick.

"These findings breathe new life into the conservation prospects for members of this flagship group."

Pure breds possibly in hiding

Genes from recently extinct species can live on in mixed ancestry creatures, but these data showed the parentage must be closer than simply a remnant of a bygone species.

In fact, the data showed that some of the breeding must have been quite recent because 30 of the 84 tortoises were under 15 years old.

And given the genetic diversity of the sample, scientists believe the minimum number of contributing purebred C. elephantopus parents would be 38.

If conservationists could locate the original purebreds, they could help revive the giant tortoises' numbers through targeted breeding, says Garrick.